The incidence of root caries within Western developed societies has reached significant levels and is expected to increase in the near future. Despite this fact, little is known about the etiology of this disease, in particular the origin and mechanism of the demineralization process involving root tissues. The aim of the present project is to remedy this situation with the anticipation that a better understanding of those factors involved in root caries formation will lead to the development of preventive measures, which at present are unavailable. Since it is evident that there are similarities as well as distinct differences in the onset of coronal caries and root caries, advantage of our previous experience in the study of coronal caries will be taken by conducting parallel experiments with the two tissues. Specifically we propose to: 1. Determine the physico-chemical properties of demineralizing media required to induce in vivo-like lesions in extracted roots. By conducting parallel experimentation with extracted tooth crowns, intrinsic differences in the properties of the enamel and the mineral in root tissues (cementum and dentine) will be ascertained, in particular how differences in prorosity and/or solubility affect lesion formation. 2. Conduct demineralization experiments on extracted tooth roots and crowns by direct colonization of oral microorganisms. The microorganisms will be S. mutans and various Actinomyces species. The experimental design will answer the question of whether there is a distinct specificity for root caries development induced by these microorganisms or whether the in vivo caries process reflects simple ecological determinants. 3. Determine if the initiation of the root caries requires some alteration of the organic matrix. This possibility will be explored by conducting direct colonization (demineralization) experiments using mutants devoid of protease activity. The wild types will be those capable of inducing caries-like lesions in root tissues in vitro and also in a gnotobiotic rat model. If proteolysis is found to play a major role in root caries formation, then the identification of non-virulent mutants may ultimately lead to their application as effector strains for replacement therapy of this disease.